Viscometers are used to measure rheological properties, e.g., viscosity, flow parameters, or other rheological properties, of fluids. A Couette viscometer can be operated to determine fluid viscosity based on a force required to turn an object in the fluid. Such a viscometer includes two co-axial cylinders (i.e., an inner cylinder and an outer cylinder) with a gap in between. A volume of the fluid to be tested is flowed in the gap and the two cylinders are rotated relative to each other. For example, the outer cylinder can be rotated at a predetermined speed while the inner cylinder is held stationary (or vice versa). The viscometer can measure the shear stress on the fluid as the outer cylinder rotates. A ratio of the shear stress to the shear rate, determined from the rotational speed of the outer cylinder, yields the viscosity of the fluid in the gap.
Sometimes, the gap between the two co-axial cylinders may need to be changed, e.g., to evaluate different fluids having different viscosities, to evaluate the same fluid at different shear rates or for other reasons. The gap may also need to be changed when evaluating well fluids, e.g., drilling fluids, production fluids, or other well fluids, that can sometimes include solid particles, e.g., drilling cuttings, pieces of the wellbore formation, or other solid particles, causing well fluid viscosities to be different at the same shear rate. Changing the gap between the two co-axial cylinders may require replacing either the inner cylinder or the outer cylinder or both for each fluid to be evaluated.
Like reference symbols in the various drawings indicate like elements.